The present invention relates to a system for controlling a coolant temperature of an internal combustion engine, and more particularly to a closed loop system for adjusting the temperature of a coolant to a target value which varies in response to operating conditions of the engine.
In widely used automotive internal combustion engines, in order to control a coolant temperature, a thermostat valve, which is designed to be opened at a predetermined temperature, for example 80.degree. C., is disposed in a coolant circulating circuit including a radiator. In this case, the coolant is kept at around the predetermined temperature of 80.degree. C.
Recently, attention has been paid to a control apparatus for keeping the coolant at different temperatures by varying the valve opening temperature of the thermostat valve in response to operating conditions of an engine.
According to this control apparatus, when an engine operates with a low load where thermal load is small with the least possibility of causing the engine to overheat, the thermostat valve opening temperature is raised to elevate the coolant temperature, thereby improving fuel economy and the exhaust gas purification. When the engine operates with a high load, the thermostat valve opening temperature is lowered to drop the coolant temperature, preventing the occurrence of knocking and enhancing "volumetric efficiency," thereby increasing power output of the engine.
One control apparatus falling into the above category is disclosed in Japanese Utility Model Application Provisional Publication No. 54-142722, which will be described referring to FIG. 1.
In a coolant passage 1, a thermostat 2 is disposed. The thermostat 2 operates as follows: When the coolant temperature rises, wax contained inside the thermostat 2 is expanded, thereby pushing a piston 3 upwardly as viewed in FIG. 1. This upward movement of the piston 3 is prevented when the piston 3 abuts a receiving portion 8 of a control rod 7 of a diaphragm device 6. Thus, a further projection of the piston 3 out of the thermostat 2 causes a valve 4 to disengage from a valve seat 12 against a return spring 5. If the control rod 7 is displaced upwards as viewed in FIG. 1, the piston 3 has to project further out of the thermostat 2 until the valve 4 starts to disengage from the valve seat 12.
Therefore, when the control rod 7 is lifted by the diaphragm 9, the valve 4 will not be opened until the coolant temperature rises further and the piston 3 is extended further. This causes an increase in the temperature of the coolant.
Intake manifold vacuum is admitted to a vacuum chamber 10 of the diaphragm device 6 acting on the diaphragm 9 in such a manner as to lift the diaphragm 9 against a diaphragm spring 11.
Since the intake manifold vacuum is high during engine operation with low load and low during engine operation with high load, the control rod 7 is lifted further, as viewed in FIG. 1, during engine operation with low load.
In this manner, the temperature at which the valve 4 is opened increases during engine operation with low load, whereas this temperature decreases during engine operation with high load. This causes the coolant temperature to rise during low load operation and drop during high load operation. If it is desired to increase the coolant temperature, a diaphragm spring 11 with a large spring force must be used because it acts on the diaphragm 9 against a reaction force which is applied to the diaphragm 9 via the control rod 7 when the valve 4 is pressed downwards against the return spring 5. On the other hand, the diaphragm 9 on which intake manifold vacuum acts against the diaphragm spring 11 must have a large pressure acting area, resulting in an increase in size and weight of each of the associated component parts. Therefore, if it is desired to set the coolant temperature high, the manufacturing cost increases. Besides, the operating life is short because the component parts are subjected to engine vibrations.
Another problem is that even though it is possible to vary the coolant temperature in response to a change in engine load, it is impossible to vary the coolant temperature in response to engine speed. Thus, this known control apparatus fails to meet a demand that the coolant temperature should drop as the engine speed increases even with the same engine load.
Referring to FIGS. 2 and 3, two well known ways of installing a thermostat are described. FIG. 2 shows a so-called "outlet control" and FIG. 3 shows a so-called "inlet control".
Referring to FIG. 2, a thermostat valve 2A is disposed in an outlet passage 15 which passes a relatively hot coolant, having cooled an engine main body 13, toward a radiator 14. A bypass passage 16, i.e., a passage bypassing the radiator 14, branches off from a portion upstream of this thermostat valve 2A and is connected to an intermediate portion of an inlet passage 18 which passes therethrough a relatively cool coolant having its heat dissipated after passing through the radiator 14 toward a water jacket (not shown) of the engine main body 13 via a water pump 17.
With this control arrangement, if the valve opening temperature of the thermostat valve 2A is set high, a difference between a temperature when the thermostat 2A is opened and a temperature when the thermostat 2A is closed increases as shown in FIG. 4, increasing the occurrence of a so-called hunting phenomenon.
Referring to FIG. 3, a thermostat valve 2B is disposed in an inlet passage 18 at a position immediately upstream of a junction where a bypass passage 16 joins with the inlet passage 18. Since the thermostat valve 2B senses the temperature of coolant resulting from mixing a relatively cool coolant having its heat dissipated via a radiator 14 with a relatively hot coolant coming from the bypass passage 16, the occurrence of the hunting phenomenon as mentioned before decreases. However, with this arrangement, because suction created by a water pump 17 acts on the thermostat valve 2B in a valve opening direction, the spring constant and the spring load of a valve return spring have to be set sufficiently high in order to prevent overcooling of coolant. To meet this requirement, the thermostat valve 2B has to use a large diaphragm device.